The invention relates to a method and apparatus for thermowave analysis using the single-beam method. It may be employed in the measurement of geometric, thermal, electronic and elastomechanical material parameters of surface layers by evaluating photothermal response signals from the surface layers. The contactless, non-destructive, method is primarily useful in coating technology as a test method for quality control.
Photothermal spectroscopic methods are known for the contactless and non-destructive determination of parameters of layers. The physical principles and the basic solutions are compiled and described, for example, in "Photoacoustic and Thermal Wave Phenomena in Semiconductors", edited by A. Mandelis for North-Holland, N.Y. 1987.
In a known method, which was developed by A. Rosencwaig, a periodic intensity modulated pumping laser stimulates a thermowave in a layer, which in turn locally modulates the refractive index, so that the modulated optical reflections (MOR) can be measured with a second, so-called test laser beam (U.S. Pat. No. 4,579,463). The signals are processed, preferably by means of the lock-in technique, in order to determine the phase and amplitude of the MOR. This method requires the use of 2 lasers of different wave lengths and, to ensure a reflection signal which can be evaluated quantitatively, the concentric arrangement of pumping beam and test beam with a vertical incidence on the layer. Moreover, the radius of the test beam should be at most half the radius of the pumping beam.
The solutions proposed in U.S. Pat. Nos. 4,634,290 and 4,636,088 and European patents 0 162 681 and 0 291 276 require, aside from a high optical precision for the defined focus location additional optical expense (telescope) for matching the waists of laser beams and an adjusting expense for placing the waists on the surface of the layer. In addition, the background noise of the test laser represents a limiting factor for the resolution capacity, so that, with the usual requirements of .delta.R/R&lt;10.sup.-6, lasers must be used, which are highly stabilized with respect to noise. Such lasers further increase the costs. When the double-beam method is used as an in situ measurement method with a large distance from the objective to the object, for example, in coating chambers, the optical adjustment and the stability are barely manageable. Single-beam methods are also known for measuring the MOR (L. Chen et al. in: Appl. Phys. Lett. 50 (1987) 1349; A. Loerincz, L. Andor in: Photoacoustic and Photothermal Phenomena, P. Hes, J. Pelzl (editors) Springer Verlag, Heidelberg, 1968, page 486.). In these single-beam methods, the reflected pumping laser radiation is detected. To separate the MOR from the reflected, modulate pumping intensity, use is made of the fact that, during the modulation by the thermowave, upper harmonic waves of the modulation frequency are formed. This enables a lock-in detection of the second upper harmonic wave. The disadvantage of this method is that the modulator of the pumping laser also produces upper harmonic waves, which distort the measurement signal. The method of the precise square wave modulation of the pumping laser proposed by A. Loerincz (Appl. Phys. B47 (1988) 40), is associated with an extremely high expenditure for the modulation at higher modulation frequencies in the MHz range (build-up times in the ps range). In this reference, the thought is also expressed of separating from the first refraction order of the reflected laser beam, which is produced by the thermowave, the MOR information contained therein by a phase-contrast method from the modulated pumping intensity. Because of the finite width of the diffraction zone, the reduction of the MOR pump upper harmonic wave ratio to 10.sup.-7 is hardly possible. The same is true for the compensation.